Scientists raised the alert status Sunday to a "watch" level, the second-highest, based on seismic activity detected January 23.

The peak is about 100 miles southwest of Anchorage, the state's most populous city.

Mount Redoubt last erupted nearly 20 years ago, in December 1989, and that eruption lasted until April 1990. Geologists think there could be an eruption "similar to or smaller than the one that occurred in 1989-90." PopSci.com: Predicting eruptions

The observatory has set up a Web camera near the summit of the volcano and another within Cook Inlet. It plans to do continuous visual surveillance, measure gas output and analyze satellite and weather-radar data.